New high temperature-resistant NiCrAl and NiCrAl+Hf felt materials

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New high temperature-resistant NiCrAl and NiCrAl+Hf felt materials

Leprince, G. / Alperine, S. / Vandenbulcke, L. / Walder, A.

Aluminization of highly porous material such as Ni-Cr metallic felts, for use as abradable seals for turboengines, has always been both a necessity for high temperature use and a technical problem. A new low pressure gas phase aluminization technique has been designed that provides both excellent penetration capabilities and high flexibility. Using this technique, aluminization of fiber 12mym in diameter of 90% porosity, Ni-21%Cr ONERA high porosity felts has been performed, as well as codeposition of aluminium and hafnium on the same base material. The final felt bulk composition is Ni-18.5%Cr-9%Al with eventual extra addition of 2wt.%Hf in the case of Al+Hf codeposition. The metallurgical structure of the fibres is of the gamma-gamma' type, with about 0.6wt.%Hf dissolved in the gamma' phase (solubility limit at 1000C), the hafnium in excess remains at the fibre periphery as Ni5Hf. The felt materials obtained in both cases behave very well as far as abradability and erosion resistance are concerned. Cyclic oxidation tests have been performed on both materials at 900, 1000, and 1100C. NiCrAl felts show excellent oxidation resistance up to 900C (lifetime greater than 1000 1h cycles) whereas hafnium-doped specimens resist as high as 1000C for the same duration. From the comparison of the oxidised specimens, some aspects of the active element effect of hafnium can be derived. NiCrAl+Hf felts seem suitable as abradable seals in turboengines up to 1000C as well as for hot gas filtration.